3.Greenland.

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D the largest island in the world修饰 GREENLAND的,具体的谓语在逗号后面,即 cover
======以下答案可供参考======
供参考答案1:
greenland, the largest island in the world, covers two million square kilometres
格陵兰，世界上最大的岛屿，占地两百万平方公里。